The present invention relates to the production of bleached cellulosic fibrous pulp. More particularly, the present invention relates to a method of producing bleached fibrous cellulosic pulp in which the chemical compounds used and formed in the various steps and stages are utilized to maximum efficiency with a minimum of loss and resulting pollution by the recovery and reuse of products previously passed as waste from pulp bleaching processes.
In the conventional Kraft process, raw cellulosic fibrous material, generally wood chips, is digested, by heating, in a pulping liquor, (white liquor) which contains sodium sulfide and sodium hydroxide as the active pulping chemicals. The digestion provides a pulp and spent pulping liquor (black liquor). The black liquor is separated from the pulp by washing in a brown stock washer and the pulp utilized as feedstock in a bleach plant for brightening and purification operations.
The black liquor is then concentrated, usually by evaporating, and the concentrated black liquor burned in a reducing furnace to yield a smelt containing primarily sodium carbonate and sodium sulfide.
The smelt is then dissolved in water to yield a raw green liquor which may be clarified to remove undissolved solids. The green liquor is then causticized, usually by treatment with CaO to convert sodium carbonate to sodium hydroxide. The resulting liquor is white liquor and is useful in the initial digestion step to provide at least a part of the pulping liquor.
The foregoing sequence or cycle is well known and is referred to herein as the digestor liquor cycle.
Bleach plant operations generally involve a sequence of brightening and purification steps which may be combined with washing steps. The brightening steps generally involve the use of bleaching agents, such as chlorine or chlorine dioxide. The purification steps involve washings and treatment with sodium hydroxide solution, caustic extraction.
A particular bleaching sequence which finds use in one mode of the present invention involves an initial bleaching of the pulp with an aqueous solution containing chlorine dioxide and chlorine, an intermediate washing, a caustic extraction using aqueous sodium hydroxide solution, a further washing, a bleaching with an aqueous solution of chlorine dioxide, another washing, a further caustic extraction using aqueous sodium hydroxide, an additional washing, a final bleaching with chlorine dioxide solution and a final washing. These are the so-called D.sub.c EDED or D/C EDED sequences.
A common source of chlorine dioxide for the bleaching operation is a chlorine dioxide generator which produces chlorine dioxide, usually as an aqueous solution of chlorine dioxide and chlorine by the reduction of a chlorate salt, e.g., sodium chlorate. Such chlorine dioxide generators utilize a feed of H.sub.2 SO.sub.4 which is reacted with a mixture of sodium chlorate with some sodium chloride to produce a chlorine dioxide bleaching product and a sodium sulfate (saltcake) by-product.